PROJECT SUMMARY/ABSTRACT Varenicline is the most effective smoking cessation therapy available. Nevertheless, most smokers using varenicline relapse within the first few months after quitting. Varenicline is hypothesized to help smokers to quit in part by reducing the reinforcing effects of smoking during the standard 1-week pre-quitting treatment phase. Learning theory and previous human and animal research support the hypothesis that a longer duration of varenicline treatment prior to the target quit date (TQD) will yield greater decreases in the number of cigarettes smoked per day before quitting, and higher long-term cessation rates, compared to standard varenicline treatment. Building on our promising preliminary clinical data, we propose to test these hypotheses with a full-scale randomized clinical trial (RCT). Four hundred treatment-seeking smokers (200 female) will be randomized to a standard run-in group (3 weeks of placebo, followed by the standard 1 week of pre-TQD varenicline) or an extended run-in group (4 weeks of pre-TQD varenicline). Both groups will receive brief individual cessation counseling and 11 weeks of post-TQD varenicline. The primary outcome measure will be bio-verified continuous abstinence at end-of-treatment (weeks 8-11 post-quit) and at long-term follow-up (weeks 8-26 and 8-52 post-quit). Hypothesized mediating mechanisms (e.g., smoking reinforcement) will be evaluated by converging behavioral, physiological, and subjective measures assessed both in the lab and using real-world, real-time electronic momentary assessments (EMA). We predict that long-term, bio-verified smoking cessation will be improved among the extended run-in group compared to the standard run-in group. We further predict the improved clinical outcomes with extended run-in varenicline will be explained (or mediated) by greater pre-quit reductions in smoking reinforcement among the extended run-in group compared to the standard run-in group. The significance of this work is clear: We aim to make best available treatment for smoking cessation even better, using a method that is ripe for dissemination and an approach that will elucidate critical mechanisms to target in the next generation of treatment enhancement.